Field of the Invention
The invention relates to an optical film, a polarizing plate and a liquid crystal display device. More specifically, the invention relates to an optical film useful in a liquid crystal display, and a polarizing plate and a liquid crystal display device containing the optical film.
Background Art
A cellulose ester film, which is represented by a cellulose acetate film, has high transparency and thus has been used as an optical film for various purposes in a liquid crystal display device. For example, a cellulose ester film is used as a polarizing plate protective film in a liquid crystal display device since adhesiveness to polyvinyl alcohol used in a polarizer may be easily secured
In recent years, a liquid crystal display device, particularly a liquid crystal display device for a middle sized or small sized equipment, undergoes drastic reduction in thickness, and thus reduction in thickness of members used therein, particularly reduction in thickness of a polarizing plate protective film (such as a protective film hating a hardcoat layer provided on a surface of a liquid crystal device, a protective film functioning as a retardation film, and an ordinary protective film having a small retardation), is being demanded. A middle sized or small sized liquid crystal display device is often exposed to severe environmental changes, for example, in outdoors, and the durability thereof under a high temperature and high humidity environment is an important capability. The progress of reduction in thickness of the polarizing plate protective film may increase the polarizer protecting function demanded per unit thickness, and thus there is more than ever a demand of a thin optical film that is capable of ensuring the durability of the polarizer under a high temperature and high humidity environment.
As a film used in a liquid crystal display device, for example, Patent Reference 1 describes that a cellulose acylate film containing a polyester polymer containing a polyester component, which is derived from a diol containing an alicyclic structure and a dicarboxylic acid derivative having an alicyclic structure, and cellulose acylate has a high tear strength.
Patent Reference 2 describes that a cellulose ester film containing an ester plasticizer having benzene carboxylic acid or phenol residual groups at both terminals thereof and having an alicyclic glycol and an alicyclic dibasic acid has an increased durability of the optical capability against humidity change.
Patent Reference 3 describes that a polyester resin modifier having cyclohexane rings or cyclohexene rings in the main chain skeleton thereof, in which the rings form a polymer through ester bonds at the 1-position and the 2-position of the ring, may enhance the moisture permeability resistance of a cellulose ester film and may suppress fluctuation of the retardation in the thickness direction Rth thereof due to humidity fluctuation.
Patent Reference 4 describes that a resin film containing a resin and an organic acid having a particular structure in an amount of from 0.1 to 20% by mass based on the resin may be improved in peeling property of the film from a metal support in the production of the resin film by solution film formation, and may also improve durability of a polarizer. Patent Reference 4 also describes that the resin film preferably contains a polycondensation polymer, such as a polyester polymer, from the standpoint of the reduction of haze, and describes 1,4-cyclohexanedicarboxylic acid as an example of the aliphatic dicarboxylic acid as a raw material monomer for synthesizing the polyester polymer.